1. Field of the Invention
The present invention relates generally to semiconductor fabrication methods and, more particularly, to a method for dynamically varying a threshold voltage of a complimentary metal oxide semiconductor (CMOS) or the like so as to reduce the turn-on threshold voltage thereof while leaving the turn-off threshold voltage thereof approximately unchanged.
2. Description of Related Art
A dynamic threshold voltage metal oxide semiconductor field effect transistor (DTMOS) is a well known type of metal oxide semiconductor field effect transistor (MOSFET). DTMOS field effect transistors require a lower threshold voltage (Vt) to turn on (when the substrate thereof is appropriately biased, as discussed below) than do standard metal oxide semiconductor field effect transistors. Thus, DTMOS field effect transistors may advantageously be used in the construction of complimentary metal oxide semiconductor (CMOS) devices and the like.
DTMOS field effect transistors have different voltage thresholds for turning the device on and off. The lower turn on voltage threshold facilitates a desirable increase in the on current of the device. The generally unchanged turn off threshold voltage facilitates normal turning off of the device.
When a DTMOS field effect transistor is turned on, the substrate is forward biased, such that a reduced threshold voltage is required for turn on. This reduced turn on threshold voltage inherently results in a corresponding increase in source to drain current (Ion). As those skilled in the art will appreciate, such increased source to drain current is desirable in many applications.
When a DTMOS field effect transistor is off, the substrate bias is zero and the device therefore maintains the original (standard MOSFET) threshold voltage. The source to drain current in the off state is approximately zero. Dynamic DTMOS field effect transistors thus provide desirably increased source to drain current for the on state (Ion), while not undesirably compromising source to drain current for the off state (Ioff).
The substrate of a DTMOS field effect transistor must thus be forward biased during turn on, so as to reduce the turn on threshold voltage. Conversely, the substrate must be zero biased during turn off, so as to keep the turn off voltage threshold approximately unchanged (the same as for a standard MOSFET).
The above described reduction in threshold voltage for turning on a DTMOS field effect transistor desirably facilitates the use of such devices in low voltage and low power applications. As those skilled in the art will appreciate, the continuing drive to increase the device density of integrated circuits necessitates that each device generally operate at lower power, so as to mitigate the heat extraction requirements of the integrate circuit. That is, it is generally necessary to reduce the power consumption of metal oxide semiconductor devices in order to increase their density on an integrated circuit chip. Reducing the voltage required to turn on individual devices tends to mitigate the overall power consumption, and consequently the heat extraction required, for a given integrated circuit chip.
One contemporary attempt to provide a DTMOS field effect transistor involves forming the device with the gate and body thereof electrically tied together. However, this approach undesirably requires that the drain voltage (Vdd) be limited to approximately 0.6V, so as to avoid inadvertent turn on of the parasitic substrate diode defined by such construction. Thus, this contemporary device is only suitable for ultra-low voltage operation.
Another contemporary attempt to provide a DTMOS field effect transistor involves the use of an additional circuit to control the body bias of the device. However, in this instance the additional circuitry inherently increases the complexity of circuits using the device. Such increased complexity undesirably increases costs, decreases yield, and decreases reliability of the integrated circuit chip using the device.
A need thus exists in the prior art to provide an improved DTMOS field effect transistor which does not require a strict limitation on the drain voltage and which does not require the use of additional circuitry.